resistance is defined as f reflecting the frictional resistance. The increase of fluid power consumption is not completely equivalent to the increase of frictional resistance, and it is equivalent to the sum of increase of frictional resistance and momentum. In Fig. 6, when C0 gets large enough, the frictional resistance decreases a little, but the momentum of flow (the intensity of longitudinal swirl flow) increases, so the performance of heat transfer increases. In general, as there is no insert in the tube, so the increase of flow resistance is not significant and the performance of optimized tube is improved